The present invention relates to a DC sputtering system, and particularly to a DC sputtering system improved in prevention of fusion of an optical recording medium and electric corrosion of a pallet revolving shaft due to the flow of negative electric charges at the time of production of the optical recording medium.
First, a related art sputtering system will be described with reference to FIGS. 5 to 8. FIG. 5 is a view showing a schematic configuration of the related art sputtering system; FIG. 6 is a schematic view illustrating a portion of the related art sputtering system, which corresponds to an improved portion of the present invention; FIG. 7 is a view illustrating fusion of a substrate of the related art sputtering system; and FIGS. 8A, 8B are views illustrating damages of a revolving shaft of a revolving type pallet, wherein FIG. 8A is a sectional view of the revolving type pallet and FIG. 8B is an enlarged sectional view of the revolving shaft.
Optical recording media (hereinafter, referred to as "optical disks") have been generally produced by a sputtering process. In the sputtering process, a target material is taken as a negative electrode, and both a substrate mounting pallet disposed opposite to the target material and a vacuum chamber housing are electrically earthed. At this time, positive ions in plasma flow toward the cathode target, and electrons flow toward the earth through the pallet, a mask fixing the substrate of the optical disk to the pallet, and the chamber housing. In general, a substrate mounting station (hereinafter, referred to as a "disk base") is provided on the surface of the pallet, and the substrate is fixed on the disk base using the mask which is composed of an inner peripheral mask and an outer peripheral mask.
Optical disks include a photomagnetic disk having a recording film made from a rare earth metal/a transition metal and a phase change disk having a recording film made from GeSbTe or the like. In general, such an optical disk is of a multi-layered structure in which a recording layer is held between dielectric layers. Further, with respect to a reproducing disk, there has been proposed a type having a multi-layered structure called a very high resolution structure, in which a mask layer and a dielectric layer are disposed between a substrate and a reproducing reflection layer. Here, as the dielectric material, there is generally used one kind selected from a group consisting of SiN, AlN, TiO, and oxides and nitrides of Si, Al, Ta, Ti and alloys thereof.
In production of an optical disk having a dielectric layer, there is a tendency to form the dielectric layer by DC sputtering. This is because the formation of the dielectric layer by RF reactive sputtering or the like takes a lot of time. Optical disks are sequentially produced using DC sputtering by repeating the following steps of mounting a substrate on a pallet with a substrate fixing mask; putting such a pallet in a film forming vacuum chamber, followed by sequential formation of a dielectric film, recording film, and the like on the substrate; dismounting the substrate from the pallet outside the vacuum chamber; and putting the pallet on which a new substrate is mounted, into the vacuum chamber.
The concrete configuration and schematic operation of a DC sputtering system used for producing optical disks will be described with reference to FIG. 5.
A DC sputtering system 50 includes a vacuum chamber 1 as a film formation chamber; a vacuum control unit 2 for controlling a vacuum condition of the interior of the vacuum chamber 1; a DC high voltage power source 3 for plasma discharging; a sputtering cathode unit 5 connected to the DC high voltage power source 3 through a power supply line 4; a pallet 6 disposed opposite to the sputtering cathode unit 5 in such a manner as to be spaced therefrom at a specific distance; and a sputter gas supply unit 7 for supplying a sputter gas such as Ar into the vacuum chamber 1.
The sputtering cathode unit 5 includes a target plate 8, a backing plate 9, and a magnet system 10. The target plate 8, which functions as a negative electrode, is made from a target material such as Si. The target plate 8 is fixedly placed on the backing plate 9. The magnet system 10 is disposed behind the backing plate 9. The target plate 8 and the pallet 6 functioning as a positive electrode constitute a pair of electrodes. A substrate 11 of an optical disk such as a photomagnetic disk on which a film is to be formed, is mounted on the pallet 6 opposite to the sputtering cathode unit 5 using an inner peripheral mask 12 and an outer peripheral mask 13, with a disk base 14 put between the substrate 11 and the pallet 6.
The operation of the DC sputtering system 50 will be described below. First, the interior of the vacuum chamber 1 is evacuated to a desired degree of vacuum, for example, to a value of 1.times.10.sup.-4 Pa or less by the vacuum control unit 2. Subsequently, a sputter gas, for example, a mixed gas of Ar and N.sub.2 is introduced into the vacuum chamber 1 to a specific pressure by the sputter gas supply unit 7. A specific negative voltage is applied, in such state, to the backing plate 9, that is, the target plate 8 from the DC high voltage power supply 3. An electric field is thus generated between the pallet 6 and the backing plate 9 which form the pair of electrodes, to generate a glow discharge. The Ar gas is ionized by glow discharge, and ions of the Ar gas bombard with the target plate 8. As a result, atoms or the like of the target material are sputtered from the target plate 8, and deposited on the surface of the substrate 11 mounted on the pallet 6 disposed opposite to the target plate 8, to thereby form a thin film made from SiN.
FIG. 6 shows a current flow path within the vacuum chamber 1 upon sputtering. As described above, when the electric field is generated between the pallet 6 and the target plate 8, ions of the Ar gas (inert gas) are bombarded with the target plate 8 and thereby target atoms are sputtered from the target plate 8. The target atoms are deposited on the surface of the substrate 11, and negative electric charge such as electrons, which are simultaneously generated, flow from the pallet 6 as an anode to the earth through the inner peripheral mask 12, outer peripheral mask 13, and the wall of the vacuum chamber 1.
However, as shown in FIG. 7, when an insulating layer 19 such as SiN film is overlappingly deposited on the pallet 6 with the increasing frequencies of working of pallet 6, the flow of negative electric charges such as electrons generated during sputtering tends to be cutoff by the deposited insulating layer 19. The flow of the negative electric charges is thus concentrated at the inner peripheral mask 12 and the outer peripheral mask 13 each of which is made from stainless steel or the like. More concretely, the negative electric charges flow toward the earth through portions where the inner peripheral mask 12 and the outer peripheral mask 13 are brought in contact with the disk base 14.
As a result, although the usual sputtering generates only a temperature rise of about 50.degree. C., the temperature of each of the inner peripheral mask 12 and the outer peripheral mask 13 exceeds a glass transition temperature (140.degree. C.) of the substrate 11. The substrate 11 is thus fused at the portions where the substrate 11 is brought in contact with the inner peripheral mask 12 and the outer peripheral mask 13, to form substrate fusion portions 20. This allows the optical disk to cause defects such as an abnormality in double refraction.
The related art system also presents another problem. Referring to FIG. 6, as the frequencies of working of the pallet 6, inner peripheral mask 12, and outer peripheral mask 13 are increased for repeatedly producing optical disks, a dielectric film, metal film, and the like tend to be deposited on the surfaces thereof, so that the pallet 6, inner peripheral mask 12, and outer peripheral mask 13 are electrically insulated. However, since the inner peripheral mask 12 and the outer peripheral mask 13 are brought in contact with a substrate mounting/dismounting unit upon mounting/dismounting of the substrate 11, the films deposited on the surfaces of the masks 12, 13 are removed, with a result that a non-insulating portion is formed on part of the surface of each mask. If DC sputtering is performed for forming a dielectric layer in such a state, the surface of the pallet 6 is insulated and thereby electrified, to be thus suppressed in incidence of electrons; however, the non-insulating portions of the surfaces of the masks 12, 13 are concentratedly bombarded with electrons.
The temperatures of the inner peripheral mask 12 and the outer peripheral mark 13 are thus excessively increased. If these temperatures exceed the melting point of the substrate 11, the substrate 11 is fused and the characteristics of the optical disk are degraded by changes in double refraction.
In the case of using a revolving type pallet, there arises the following problem.
FIG. 8(a) is a sectional view of a revolving type pallet. The pallet 6 is fixed on a pallet holder 21 rotatably held by a carrier 15 through a bearing 22 and a bearing housing 23. The revolving force is introduced through a magnet coupling 24.
The revolving type pallet uses the bearing 22 as a revolving portion as described above, so that electrons impinging on the pallet 6 pass through the bearing housing 23, and flow toward the bearing housing 23 through an outer race of the bearing, balls, and inner race of the bearing, and they flow toward the earth through the carrier 15. At this time, there occurs electric interruption between the balls and the inner and outer races of the bearing 22 because of the presence of grease therebetween and revolution of the balls, so that if the carrier 15 is not insulated, arc discharges are generated at portions shown in FIG. 8B. The portions at which arc discharges have generated are susceptible to electric corrosion, causing a revolving failure of the pallet 6. The same is true for the case that a rotating mechanism is provided on a chamber main body.